There is strong interest is providing direct transmission on high capacity data transmission systems within and between all types of network switches. Switches for the purpose of this discussion being: switch modules of a larger switch; PBX's; stand alone end office switches and network access switches.
By way of example of such a need, the prior art toll network is composed of expensive toll switches which serve tandem calls and provide toll network access to end switches of the Public Switched Telephone Network (PSTN). As an alternative to such arrangements, U.S. Pat. No. 5,345,446 which issued on Sep. 6, 1994, proposes use of Asynchronous Transfer Mode (ATM) cells, or ATM-like cells, to communicate digitized voice and digital data signals between pairs of access switches of the PSTN. Such communication is provided by Permanent Virtual Circuits (PVC) through the network. Permanent Virtual Circuits are activated and deactivated as a function of the traffic between offices. The proposed ATM transmission standard packs data into independent cells of a time frame, e.g., a time frame of 125 .mu.sec. The cells are transmitted serially without the presence of any framing signals; and the number of cells in any 125.mu. second period is dependent on the bandwidth of the ATM transmission facility. In the prior art example of U.S. Pat. No. 5,345,446, each cell is 53 bytes long and comprises a 5 byte header and a 48 byte payload. In the case of DSO 64 kilobits/second PCM signals, in the case of DSO channels, each byte of the 48 byte payload carries a sample signal of a different PCM channel. Higher rate PCM channels can be accommodated in such cells by assigning a number of bytes in a cell, e.g., by assigning 6 bytes in a cell to 384 kilobits/second channels. Similarly, lower rate PCM channels can be accommodated by transmitting their samples in alternate 125.mu. sec. periods. The five byte headers identify a particular Permanent Virtual Connection (PVC) in the network and indirectly define the assignment of the payload bytes to calls being served.
A problem faced in communicating ATM cells between communication network end points, e.g., between switch modules of a 5ESS.RTM. switch; and between end offices, are the tasks of composing and decomposing ATM cells to permit selective real-time transfer of payload data between ATM channel facilities and between an ATM channel and the various formatted synchronous PCM channels served by a network access switch. U.S. Pat. No. 5,345,446, demonstrates use of an Asynchronous Transfer Mode Interface Unit (ATMU) between a SONET/ATM link connected to a Common Broadband Platform and a Network Control and Timing (NCT) link connected to a digital access switch. The NCT link, in turn communicates with other PCM facilities, e.g., DSO facilities, DS1 facilities, through the time slot interchange facility of a connected access switch. In this prior art arrangement, communication between ATM cells and synchronous PCM channels requires double conversion of format in both directions of communication and introduces concomitant transmission delays.